The invention relates to a method for controlling a friction clutch for connecting first and second rotatable axles in a vehicle, e.g. an input shaft of a transmission and a crankshaft of an engine. The invention also relates to a computer program, a computer readable medium, and a controller for performing the steps of the method.
As is known, a clutch is used in a vehicle for connecting a first rotatable axle in the form of an input shaft of a transmission, and second rotatable axle in the form of a crankshaft of an engine. Dry friction clutches are widely used as master clutches in manual, automated and automatic mechanically engaged vehicle transmissions.
In such a clutch, a first friction part in the form of a disc is rotationally connected to the transmission input shaft. The disc is arranged to be pressed axially and clamped by a spring system between second and third friction parts in the form of two friction plates which are connected to the engine crankshaft. In some clutches two friction discs and or three plates are provided. The third friction part is fixed to the crankshaft while the first and second friction parts are locked in the rotational direction to the transmission input shaft and the crankshaft, respectively, while movable in a non-rotational direction, e.g. axially. Thereby the second friction part can move between positions of engagement and disengagement with the first friction part so as to engage and disengage to clutch.
The spring system comprises a spring element which is locked to the crankshaft, in the rotational direction thereof. The spring element is adapted to bias the second friction part into engagement with the first friction part, so as for the first friction part to be clamped between the second and third friction parts. Upon engagement, torque can be transferred between engine and transmission via friction between the first friction part and the second and third friction parts.
A clutch of the kind referred to here also comprises an actuator, which can be for example pneumatic, hydraulic, or of a non-fluid type, e.g. an electromechanical actuator. The actuator comprises a movable actuator part which is engageable with the spring element. The movable actuator part is adapted to move, usually axially, to provide an actuation force deforming the spring element. Thereby the second friction part is disengaged from the first friction part to disconnect the crankshaft and the transmission input shaft. Thus when disengaging, the spring system is counteracted by an external force exerted by the clutch actuator. Normally, a portion of the actuator is non-rotating, and a release bearing transfers the actuation force to the rotating spring system. The actuator is usually attached to the transmission.
There are two main types of dry friction clutches, characterised by the spring system, namely push-type clutches and pull-type clutches. In push-type clutches, the axial actuation force of the actuator via the release bearing is directed towards the friction parts, and the actuator pushes the spring element out of action. In pull-type clutches the actuator pulls the spring element out of action, and the axial actuator force on the release bearing is directed away from the friction parts. A push-type clutch and a pull-type clutch are shown in FIG. 1 and FIG. 2, respectively, in WO03/019026 A1.
Pull-type clutches may have lower actuation force than push-type clutches for the same clamping force. Hence, pull-type clutches are common in high-torque vehicle applications, e.g., heavy trucks and buses. Push-type clutches, on the other hand, allow a considerably less complicated assembly when joining the transmission to the engine. Since the actuation force is directed towards the clutch, simply axial contact between the movable actuator part and spring system is sufficient. In a pull-type clutch, the movable actuator part needs to be displaced to partly enter inside the spring system. Then, it will be joined with the spring system in a way that enables actuation in the opposite direction, away from the friction parts. A pull-type clutch with an actuator and a connection between release bearing and spring system is shown in FIG. 1 in DE 19716600 A1.
It follows that, at assembly, when joining of engine and transmission with a pull-type clutch, in order for the movable actuator part to mate with the spring system properly, the movable actuator part must be kept in a position extending towards the friction parts.
DE102013217592A1 suggests for this purpose a blocking device adapted to assume a blocking position in which it prevents, restricts, or blocks the movement of the movable actuator part towards the disengagement position. In a non-blocking position, in which it could be still mounted to the clutch or completely removed, the blocking device allows the movement of the movable actuator part towards the disengagement position. While this system provides an advantageous assistance when assembling a pull-type clutch, it is important that the blocking device is removed from the blocking position after the assembly, and that it is not returned to the blocking position during normal operation of the clutch. Failure to remove the blocking device, e.g. after vehicle service or in a new vehicle, before clutch operation can result in damage to the clutch.
It is desirable to ensure correct operation of a vehicle clutch. It is also desirable to ensure correct operation of a vehicle clutch after assembly of the clutch. It is also desirable to reduce or eliminate the risk of damage caused by a blocking device for clutch assembly left in the blocking position at normal use of the clutch.
According to an aspect of the invention, a method is provided for controlling a friction clutch for connecting first and second rotatable axles in a vehicle, where the clutch comprises                a first friction part which is connected to the first axle,        a second friction part which is connected to the second axle,        a spring element which is adapted to bias the second friction part into engagement with the first friction part to connect the first and second axles, and        an actuator comprising a movable actuator part which is engageable with the spring element, and adapted to move, upon being subjected to an actuation force from an actuator control system, towards a disengagement position, thereby deforming the spring element to disengage the second friction part from the first friction part to disconnect the first and second axles, the method comprising the steps of        subjecting the movable actuator part to a test force urging the movable actuator part towards the disengagement position, the test force being smaller than the actuation force, and        subsequently or simultaneously to subjecting the movable actuator part to the test force, determining a position of the movable actuator part.        
Determining the position of the movable actuator part upon subjecting it to the test force can be used to provide an indication whether or not the clutch is in order for normal use. If the clutch is not in order for normal use, further use of the clutch can be disallowed or warned against. Thereby improper use of the clutch possibly resulting in damage can be avoided. Since the test force is smaller than the actuation force, the invention makes it possible to use the test force to determine whether the clutch can operate normally, without subjecting it to a force which is large enough to cause damage in case there is a condition in the clutch which can cause damage during normal use. Determining the position of the movable actuator part upon application of the test force can be used for providing an indication of whether the clutch can operate normally, which indication is easy to register, e.g. by a vehicle controller.
Preferably, the position of the movable actuator part is determined upon subjecting the movable actuator part to the test force. Determining the position of the movable actuator part is preferably carried out before subjecting the movable actuator to any further force, subsequent to the test force, by means of the actuator control system. Where determining the position of the movable actuator part is carried out simultaneously to subjecting the movable actuator part to the test force, preferably the position determination is carried out after initiation of the application of the test force to the movable actuator part. Thus, since the test force would normally be applied during a time interval, which however can be short, the position determination should be carried out after the beginning of said time interval. This may allow a movement of the movable actuator part before the position determination.
The clutch can be a pull-type clutch. As stated, the clutch can comprise a blocking device adapted to assume a blocking position in which it prevents the movement of the movable actuator part towards the disengagement position, and a non-blocking position in which it allows the movement of the movable actuator part towards the disengagement position.
Thereby, the method can further comprise determining, based on the determination of the position of the movable actuator part, whether the blocking device is in the blocking position. Thereby, a rigid test can be provided to detect whether the blocking device is in the blocking position.
Preferably, the method comprises comparing the determined position to a fixed reference position. Since the positions of the movable actuator part in engaged and disengaged conditions of the clutch may vary depending on the wear of the friction parts, applying the test force, determining the movable actuator part position, and comparing the latter to the fixed reference position, can make it possible to reliably provide an indication whether or not the clutch is in order for normal use. For example, where the clutch comprises a blocking device as mentioned above, the reference position can be as close to the disengagement position as the movable actuator part can be when the blocking device is in the blocking position. If the movable actuator part position, determined upon application of the test force, is closer to the disengagement position than the reference position, this indicates that the blocking device is in the non-blocking position, and the clutch can be operated normally. This form of reference position is herein also denoted as an obstruction position.
Thus, where the clutch comprises a blocking device as mentioned above, the method can comprise determining whether the movable actuator part position, determined subsequently or simultaneously to subjecting the movable actuator part to the test force, is between the obstruction position and the disengagement position. It should be noted that in case the movable actuator part is, before the test force is applied, in a position which is further away from the disengagement position than the obstruction position, the movable actuator part could move, albeit, if the blocking device is in the blocking position, not further than to the obstruction position. Nevertheless, determining whether the movable actuator part can move to a position between the obstruction position and the disengagement position will provide a clear indication whether the blocking device is in the blocking position.
Preferably, if the blocking device is in the non-blocking position, the test force can move the movable actuator part so that the movable actuator part becomes biased against the spring element. In many pull-type clutch designs, the movable actuator part is biased against the spring element during normal use of the clutch, in particular at the disengagement position of the movable actuator part. However, where the clutch comprises a blocking device as mentioned above, the movable actuator part may not biased against the spring element in the obstruction position. The reason could be that the movable actuator part and the spring element are arranged with a snap arrangement for facilitating the assembly of the clutch. For example, where the spring element is of a diaphragm type, presenting a centred circular hole, the movable actuator part could present a bevelled snap flange. Thereby, during clutch assembly, when the blocking device is in the blocking position, the snap flange passes through the spring element hole, deforming the spring element slightly, after which the spring element “snaps” back to a non-deformed position. As a result, there will be a distance between the snap flange and the spring element in the obstruction position. If the blocking device is thereafter removed, the test force can move the movable actuator part to a position, in which the snap flange moves into contact with, and becomes biased against the spring element.
Preferably, the method comprises determining, based on the movable actuator position determined subsequently or simultaneously to subjecting the movable actuator to the test force, whether to prevent, or disallow, the actuator control system to subject the movable actuator part to the actuation force. This makes it possible to ensure that the clutch is not operated during a suspected operational disorder, for example the blocking device being in the blocking position, whereby damage can be avoided.
The method can comprise determining, based on the movable actuator position determined subsequently or simultaneously to subjecting the movable actuator to the test force, whether to issue an alert signal for an operator of the clutch. This can be used to alert a driver of the vehicle of the clutch disorder so as for this person to avoid any attempt to disengage the clutch.
Preferably, the movable actuator part is subjected to the test force using the actuator control system. Thereby, no substantial additional equipment is needed to carry out the method.
Instead the existing clutch control system can be used, e.g. with an adjustable pressure control to effectuate the differentiation of the test force to the actuation force. Also, no special equipment is needed for the position determination step. Rather, determining the position of the movable actuator part can be carried out by means of a position sensor, which is readily available in a modern vehicle clutch.
Preferably, where the actuator control system comprises a fluid pressure system, e.g. a pneumatic system or a hydraulic system, the test force is controlled at least partly based on feedback from a pressure sensor in the fluid pressure system. Thus, the test force can be controlled by controlling the pressure in the fluid pressure system, based on feedback from the pressure sensor. This provides an arrangement which is simple to implement in existing vehicle clutch control systems. The test force can for example be controlled by means of a valve in the fluid pressure system, the pressure sensor being located between the valve and the actuator, the valve being controlled at least partly based on the feedback from the pressure sensor. Of course the pressure sensor being located between the valve and the actuator, does not necessarily mean that it is spatially located between the valve and the actuator. Rather the meaning is that it is functionally, in the fluid pressure system, located between the valve and the actuator.
In some embodiments, the test force is at least 1%, preferably at least 3%, more preferably at least 5%, most preferably at least 10%, of the actuation force. In some embodiments, the test force is not higher than 80%), preferably not higher than 60%, more preferably not higher than 30%), most preferably not higher than 15%, of the actuation force. The test force level has to be high enough to ensure that the inventive test can provide a movement of the clutch actuator. However, the test force should be low enough to not cause damage in the clutch, e.g. in the case of the blocking device being in the blocking position. An advantageous interval for the test force to actuation force ratio is 5-30%>, preferably 10-15%.
In some embodiments, the method comprises determining, at least partly based on the determination of the position of the movable actuator part, whether the movable actuator part moves in response to the test force. If the movable actuator part does not move in response to the test force, this can serve as an indication that the clutch is not in order for normal use. Preferably, the position, determined subsequently or simultaneously to the step of subjecting the movable actuator part to the test force, is a second position, and the method further comprises determining, before the step of subjecting the movable actuator part to the test force, a first position of the movable actuator part, the step of determining whether the movable actuator part moves in response to the test force being carried out partly based on the first position. In such embodiments, a movement of the movable actuator part determined based on the first and second positions, can be done without the need for providing a reference position, or an absolute position. Instead the relation between the first and second positions will allow the movement determination. This can be advantageous in clutches where positions of the movable actuator part at various operational conditions of the clutch, e.g. engaged or disengaged, depend on the level of wear of the friction parts.
Preferably, the step of determining whether the movable actuator part moves in response to the test force comprises determining whether the movable actuator part moves towards the disengagement position. In the case of detecting the presence of the blocking device in the blocking position, a movement of the movable actuator part towards the disengagement position will provide a safe indication that the blocking device is removed from the blocking position.